1. Field of the Invention
This invention relates to wireless voice and data communications, and more particularly to methods and systems to select an antenna in wireless transmission communication systems.
2. Description of the Prior Art
There are several wireless communication standards. For example, the Institute of Electrical and Electronic Engineers (IEEE) has established a wireless standard, IEEE 802.16e. The IEEE 802.16e standard (IEEE 802.16e) outlines Media Access Control (MAC) and Physical Layer (PHY) specifications for wireless networks. The specification of the IEEE 802.16e addresses transmission of data in wireless networks. In particular, the IEEE 802.16e standard addresses communication in wireless asynchronous transfer mode (ATM) systems, covering frequencies of operation between 2.5 gigahertz (GHz) and 6 GHz. As is known in the art, IEEE 802.16e uses a modulation method called orthogonal frequency-division multiplexing access (OFDMA), which allows communication to occur at extremely high data speeds by transmitting data over multiple frequency channels over a wide frequency range.
The IEEE 802.16e specification includes mechanisms to maximize data transmission and reception reliability in packet transmission. Typically, several processes are performed in the receiver to ultimately receive the transmitted data, including: synchronization, channel estimation and equalization, OFDM demodulation (e.g., by Fast Fourier Transforms), demapping, de-interleaving, decoding, and descrambling. The more relevant sections of the IEEE 802.16e specification for the discussion below include sections 8.4.2, 8.4.4, 8.4.6, and 8.4.9, which are hereby incorporated by reference.
The antennae used for the transmission or receipt of these packets play a crucial role. An antenna is a device that transmits or receives electromagnetic wave signals. The signals may be, for example, received by another antenna located at a proximate or a distant location. The antennae may be mounted within, for example, a transmission or receiving device in a wireless communication network. Some examples of transmission devices include wireless base station or access point devices, and mobile station devices. One example wireless communication network system is disclosed in the Mobile WiMAX Technical Overview and Performance Evaluation document prepared on behalf of the WiMAX Forum and published on Feb. 21, 2006, which is hereby incorporated by reference.
The method of selecting an antenna from a plurality of antennae to attain a superior channel is very important in maintaining a communication link. In wireless communications, multiple-antenna can significantly improve the system robustness and throughput. Typically, a receiver has a default mechanism to select a new antenna when the current antenna has an unacceptable level of quality and continues operations by using the new antenna until it degrades unacceptably. Unfortunately, since mobile station devices usually have a single radio-frequency base band (RF-BB) path, it extremely difficult for a mobile station device to determine which antenna offers the best channel without actually using the antenna. Thus, it is likely that the mobile station device will perform worse after the switch to the new antenna. When this occurs, the mobile station device often iterates through untried antennae searching for an antenna that would work. Such antenna search iterations can result in a lengthy interval of service outage for the mobile station device.
Ideally, an antenna selection (AS) scheme should choose an antenna with the best channel quality from all the available antennae. However, since in many embodiments only one RF-BB path exists in the transceiver, it is difficult to simultaneously monitor the channel quality of all the antennae. A practical way is to choose one antenna until some quality indicator, such as bit error rate (BER), frame error rate (FER), or receipt Not Acknowledged (NACK) rate, is worse than some acceptable level, then switching to another antenna. This kind of scheme does not exploit the full benefit of antenna selection diversity because its antenna selection is passive and not optimal.
Normally, an antenna selection is based on the value of a quality indicator, related either to the antenna used and/or the communication channel (e.g., an antenna gain figure, a cyclical redundancy check (CRC) parameter, a receive signal strength indicator (RSSI), a carrier to interference+noise ratio (CINR), a signal-to-noise figure, a bit error rate, a symbol error rate, or an equivalent quality indicator). The types of quality indicators may also be divided into two major categories: (1) those which are designed to monitor signal transmissions and select an antenna as the signal is received and (2) those which are designed to monitor signal transmissions and select an antenna after the signal is received.
FIG. 1 illustrates a flowchart of a method to switch among a plurality of antennae based on a quality indicator, according to the prior art. The sequence starts in operation 102. Operation 104 is next and includes monitoring over time a quality indicator relating to the use of a first antenna. Operation 106 is next and includes using the first antenna if the quality indicator does not fail, and if the quality indicator fails a pre-defined value, switching to another antenna. The method ends in operation 108. In the prior art, it should be noted that the number of available antennae is perhaps very small, so that antennae are typically chosen in one standard sequence. There is no provision in a prior art antenna selection method or module for optionally selecting the next antenna based in part on any quality indicator predicting the condition or reliability of other possible antenna choices.
In a time division multiplexed access (TDMA) wireless system, for example, the antenna selection is controlled by software or logic circuitry. In this system, a CRC parameter or an equivalent is generally used to select an antenna after the signal is received. The CRC is based on polynomial division in which each bit of a packet of data represents one coefficient of a polynomial. The polynomial is then divided by a pre-programmed polynomial to yield a quotient polynomial and in some cases a remainder polynomial. When the division yields a remainder polynomial, the system assumes that a transmission error occurred and selects another antenna. If, however, the division does not yield a remainder polynomial, the system assumes no transmission errors occurred and therefore does not select another antenna.
One example of a current antenna selection process is illustrated in FIG. 2. Comparator 202 receives inputs CRC 204 and CRC threshold 206 as inputs and then produces a result 204 coupled to the next frame antenna selection module 206. A CRC error rate that produces good speech quality is used as a threshold for selecting an appropriate antenna. If the present antenna provides a CRC error that is below the threshold value, no antenna switching occurs. However, when the CRC error rate rises above the threshold value, another antenna is selected.
While the CRC comparison provides an antenna selection by monitoring transmitted data, it has disadvantages. Its primary shortcoming is that antenna selections are not made in real time. The present antenna selected is based on a previous CRC comparison, which does not change until the antenna receives a poor quality signal. The time delay that exists between receiving an incoming signal and selecting another antenna makes the selection process susceptible to errors due to interference. A CRC selection may be accurate if a transmitter or receiver is stationary or moves at a slow rate of speed, because the communication environment is subject only to slight variations in time. However, when a transmitter or receiver moves at a high rate of speed, this time delayed process may be ineffective because it may not react to a changing environment and thus, it may be susceptible to interference.
Another technique for antenna selection monitors signal transmissions and selects an antenna as the signals are received. Preamble diversity switching is an example of a system that provides real-time measurements and real-time antenna selection. Preamble diversity switching sequentially measures the receive signal strength of a diversity of antennae at the beginning of each extended preamble. The receive signal levels of each antenna, which are the receive signal strength indicators (RSSI), are stored and compared. The antenna with the higher RSSI value is selected. When the RSSI value associated with another antenna is higher, that antenna is then selected.
The preamble antenna selection process provides the benefit of selecting an antenna as signals are received. The system is less affected by rapid environmental change. However, problems arise when differences between RSSI values are insignificant. When insignificant differences exist, the system may experience some uncertainty when selecting an antenna. This is simply because minor differences in RSSI values indicate that the signal qualities received by the antennae are similar and therefore, an antenna selection will not necessarily improve the receiving quality. Therefore, a conventional preamble diversity switching process may not be the best method for selecting an antenna.
It is not unusual for an antenna to receive a signal across a fading channel. Multiple antennae are typically used in communication systems to provide another option to turn to, in the event of poor signal reception due to a fading channel, so that a good channel with no fading can be found. Some examples of a fading channel include phase shift in the signal and multi-path interference errors. The RF energy that is transmitted between antennae can experience destructive and constructive interference due to multiple paths taken by the energy with multiple delays on the way to a receive antenna. The interference can cause a receive antenna to receive a packet in error or to miss a packet entirely.
Ideally, an antenna selection is used when a particular channel is fading due to multi-path effects so that changing from one antenna to another antenna provides another communication channel that in all probability is not fading. Trying and testing of multiple antennae typically takes place during a preamble, header, or training portion of the packet. The preamble is examined rather than the data so that no data are lost while the different antennae are being tested.
There are several reasons why this approach has been difficult to implement for the IEEE 802.16e standard, and for any other high data rate radio system. First, the packet preamble in IEEE 802.16e is quite short, because a short preamble is desirable in any high data rate communications system in order to keep the efficiency of the communications system high. If the preamble is a long period in time, then the efficiency is low. While having a short preamble is good for efficiency, the short preamble reduces the time available to test the antennae. Switching from one antenna to another antenna also takes a certain time based on the physical constraints of driving electrical switches. In addition, there is a minimum time needed to measure the signal from a given antenna to effectively determine the quality of the signal. When the measurement time (i.e., no more than the duration of the preamble) is very short, a very poor estimate of the quality may be obtained if many antennae are tested.
At higher frequencies, the signal is more directional and is more easily interrupted by relative movements of the transmitter and/or receiver. Furthermore, at higher frequencies the amount of data transmitted in a unit of time increases, creating a need to avoid or minimize interruptions caused by antenna failure. Therefore, an antenna selection should be optimized as much as possible to deal with the greater vulnerabilities and consequences of higher frequency and faster data transmission environments.
In view of the foregoing, what is needed is an improved method and system to more closely optimize the selection of an antenna from a plurality of antennae when an antenna and/or channel is degrading during use. Wideband wireless antenna applications and narrowband wireless antenna applications could both benefit from such methods and systems.